In all pulping processes in which there is some chemical treatment of comminuted cellulosic fibrous material (such as wood chips), it is necessary to impregnate the chips with treatment liquid in order for the treatment to be effective. In order for uniform treatment to be possible, however, it is necessary to remove much of the air which is contained within the wood chips, since the air acts as a barrier to chemical diffusion. This is commonly dealt with in many pulping processes by steaming the chips to pretreat them, however there are some circumstances in which presteaming is undesirable. For instance in the production of mechanical pulps utilizing sulphite treatment, presteaming has a tendency to darken the final pulp produced therefore it should be avoided.
A pulping process called the Valamo method has been used on a laboratory scale and in a batch process in order to effect deaeration of the pulp (so that it may be impregnated) by subjecting it to cycles of pressure (for example 2-5 atmospheres). In this method, the submerged wood in a batch vessel is subjected to a pressure build-up cycle of about 10 seconds, and then a pressure release cycle of about 5 seconds. This effectively pumps the air out of the chips in about 5-6 cycles. While that procedure can be useful in removing air, it is difficult to assimilate with conventional continuous pulping processes, which is desirable for maximum commercialization.
According to the present invention a method and apparatus are provided which provide an alternative to presteaming for air removal during impregnation with treatment chemicals, and particularly a method and apparatus that can be useful as an alternative to presteaming in sulphite treatment during the production of mechanical pulps. According to the basic feature of the present invention, it is possible to continuously subject the wood chips. while entrained in treatment liquid (such as a sulphite liquid) to pressure pulsations to effectively pump the air out and impregnate the chips with chemicals.
The preferred manner in which the chips are deaerated and impregnated according to the present invention is by passing the chips up and down in a generally serpentine generally vertically oriented path so that the chips are subjected to continuous pressure build-up as they are moved downwardly, and continuous pressure release as they move upwardly. Typically the height of each leg of the path would be about 90 feet to achieve a desirable pressure change. Since it is desirable to provide a different time for pressure release than build-up during the alternating pressure pulsation cycles, the cross-sectional area of the serpentine conduit is controlled so that for the upwardly extending legs the area is significantly less (e.g. about half) of the cross-sectional area in the downwardly extending legs so that the pressure build-up leg residence time is about twice as great as the pressure release residence time.
The exemplary apparatus that is utilized according to the invention comprises a conventional high pressure feeder associated with a high pressure pump, the pump preferably being one having a fly wheel. Chips in a chute are led to the high pressure feeder, in which they are entrained with the treatment liquor (e.g. sulphite liquor during the production of mechanical pulp). The high pressure feeder is connected to a first end of the serpentine vertically extending path, while the opposite end thereof is connected to a drainer (which is operatively connected to the pump too), and the chips being discharged from the drainer pass to a screw, and then to a refiner.
Utilizing the invention it is possible to quickly, effectively, and simply deaerate chips, and impregnate them with treatment liquor, while avoiding the use of presteaming, and in a continuous manner.